Field of the Invention
The present invention relates to a developing apparatus including a discharge outlet configured to discharge developer from a developing container.
Description of the Related Art
There has been widely used a developing apparatus configured to circulate two-component developer as a mixture of magnetic carrier and nonmagnetic toner while agitating and conveying the two-component developer by using a pair of conveying screws interposing a partition wall therebetween and arranged along a longitudinal direction of a developing container.
In the above-mentioned developing apparatus, fresh nonmagnetic toner is supplied with consumption in the nonmagnetic toner. However, it is known that, if old magnetic carrier continues to be circulated in the developing container, charging performance of the magnetic carrier to the nonmagnetic toner is gradually reduced. Accordingly, also as the control of the magnetic carrier in the developing container, there has been adopted control of maintaining constant charging performance of the magnetic carrier in the developing container in such a manner that the old magnetic carrier is discharged little by little and a fresh magnetic carrier is supplied by an amount corresponding to the discharge amount.
Japanese Patent Application Laid-Open No. 2002-72686 (Patent Literature 1) discloses a developing apparatus in which two-component developer for replenishment with an appropriate mix of the magnetic carrier and the nonmagnetic toner is supplied into the developing apparatus along with image formation, and thus the old magnetic carrier in the developing container is renewed little by little. Here, with reference to FIG. 2, the two-component developer circulating in the developing container is discharged little by little through a discharge opening 53 provided in an end in a conveying direction of a conveying screw 46.
The conveying screw 46 comprising a main spiral portion and a sub-spiral portion 50 connected to a downstream side of the main spiral portion. The main spiral portion conveys the two-component developer in a circulating direction and sends the two-component developer to the discharge opening 53. The sub-spiral portion 50 conveys the developer by the rotation in a direction opposite to a conveying direction of the main spiral portion. The sub-spiral portion 50 forces back most of the two-component developer that is conveyed by the main spiral portion to move toward the discharge opening 53 so that the sub-spiral portion 50 prevents the two-component developer from being excessively discharged through the discharge opening 53.
For the conveying screw disclosed in Patent Literature 1, an amount of the two-component developer falling into the discharge opening is fluctuated depending on a rotation angle of the sub-spiral portion. This is because a larger amount of the two-component developer falls into the discharge opening at a rotation position at which a root of the screw exposed at an end of the sub-spiral portion is faced upward than at a rotation position at which the root of the screw is faced downward. This phenomenon may cause an unstable discharge amount.
In view of the above, the configuration as described in Japanese Patent Application Laid-Open No. 2010-256701 (Patent Literature 2) is disclosed. Patent Literature 2 proposes, as illustrated in FIG. 4, the structure in which a disk portion 51 connected to an end portion, opposed to the discharge opening 53, of the sub-spiral portion 50 prevents the root of the screw from being exposed to the discharge opening 53. The sub-spiral portion 50 provided with the disk portion 51 is used.
However, the configuration disclosed in Patent Literature 2 has the following problem. That is, though discharge of the developer is essentially required to be stopped in a case where the amount of the developer in the developing container is reduced, the developer is discharged more than necessary. As a result, the developer in the developing apparatus is extremely reduced so that a quality of an output product may be degraded.
The above-mentioned problem results from the following phenomenon. That is, at a joining portion joining the sub-spiral portion 50 and the disk portion 51 together, a thickness of a blade of the sub-spiral portion 50 and a thickness of the disk portion 51 are added in the conveying direction. Accordingly, apparently, there is a portion having a locally increased blade thickness. At the portion having the locally increased blade thickness, an excessive amount of the developer is thrown up toward a developer discharge outlet, and hence discharge of the developer is not stopped.
The reason will be described below. A small amount of the developer present between an outermost circumferential surface of the sub-spiral portion 50 and an inner surface of the developing container is conveyed in a circumferential direction by the rotation of the conveying screw 46. Similarly, a small amount of the developer present between an outermost circumferential surface of the disk portion 51 and the inner surface of the developing container is also continuously conveyed in the circumferential direction by the rotation of the conveying screw 46. The developer is conveyed in the circumferential direction of the conveying screw 46 by a frictional force generated between the outermost circumferential surface of the sub-spiral portion 50 and the developer and a frictional force generated between the outermost circumferential surface of the disk portion 51 and the developer. Accordingly, in a case where the conveying screw 46 includes a portion in which the area of the outermost circumferential surface of the blade portion of the conveying screw 46 is locally increased, a force of conveying the developer in the circumferential direction is increased at this portion of the conveying screw 46, and thus the developer is thrown up at a position at which the developer is moved away from an inner wall of the developing container. A part of the thrown-up developer climbs over the disk portion 51 and falls toward a discharge screw 49, and hence discharge of the developer is accelerated.
That is, a developer throwing-up phenomenon due to the joining portion joining the sub-spiral portion 50 and the disk portion 51 occurs. The throwing-up phenomenon occurs as long as the developer is present in a slight gap between a bottom surface of the developing container and the joining portion joining the sub-spiral portion 50 and the disk portion 51 even when only a small amount of the developer is present on an upstream side in a direction of conveying the developer by the sub-spiral portion 50. Thus, though discharge of the developer is essentially required to be stopped in a case where the amount of the developer in the developing container is reduced, the developer is discharged more than necessary. As a result, the developer is extremely reduced so that the quality of an output product may be degraded.